1. Field of the Invention
This invention relates to a novel system for the production of tubes of thin gauge thermoplastics and, more specifically, relates to a system for continuously preheating a continuously advancing flattened tube of thermoplastic material in an area which is subsequently subjected to slitting and sealing whereby a plurality of tubes are produced from a single tube.
2. Description of the Prior Art
U.S. Pat. No. 3,156,010, the disclosure of which is incorporated herein by reference, discloses a method and apparatus which may be employed for slit-sealing a continuously advancing flattened tube of a thermoplastic material whereby a plurality of individual tubes are formed. As the patent discloses, a single, continuously advancing, flattened tube is passed directly into the path of a heated wire, the heated wire serving to simultaneously slit and seal the edges of the tubes being formed. Instead of utilizing a heated wire other slit-seal elements may be employed such as a hot knife or razor to effect the slit-sealing of the tube. When the thermoplastic film which forms the tube which is being treated, i.e., slit-sealed, is of substantially uniform gauge, such prior art slit-sealing devices perform generally satisfactorily. However, when the gauge of the film is not uniform and, for example, if the film is a profile extrusion characterized by having ribs (as for example the ribbed film product disclosed in U.S. Pat. No. 3,984,047, the disclosure of which is incorporated herein by reference) or thickened areas running longitudinally thereof the ribs serve to prevent (mechanically) the superposed layers of film from being completely sealed at their edges when such prior art slit-seal devices are employed. The individual ribs running against and across the slit-seal element cause the film layers to flutter forcing them further apart and results in poor seal formation evidenced by pin holes in the seal or even resulting in the formation of extremely weak or open seals, i.e., lengths of film in the seal area which are completely unsealed.